The subject of the invention is improvements in shock absorbers and shock absorber jacks, particularly of the type equipping aerodyne landing gears.
Many shock absorbers of this type are already known, which comprise a shock absorber shaft mounted to slide in a shock absorber cylinder, either or both of these components containing one or two chambers of gas under pressure, serving as a pneumatic spring returning the device to the position of equilibrium after damping the displacement of the shaft in the cylinder, resulting in a dissipation of energy by throttling a hydraulic fluid through orifices calibrated for normal landing speeds, on the order of 3 m/s, making it possible to absorb the energy on impact.
Some of these known devices also comprise valves equipped with throttling orifices with a variable section depending on the rate of penetration or the direction of passage of the fluid, as well as means for braking at the end of the expansion of the shock absorber, in order to make the latter effective as the aerodyne taxis over unprepared ground or over paved runways, or to absorb certain peaks of force; in order to absorb the effect of the resonance phenomenon on the ground, which manifests itself on helicopters at the limit of lift, just before take-off, and in order to prevent the device from extending abruptly, with a shock when it reaches the end of its expanded position.
But these known devices prove not to be adapted to emergency or crash landings of the aerodyne, at vertical speeds on the order of 10 to 12 m/s, for which the compression throttling orifices appear to be undersized, so that the shock absorber behaves as a quasi-rigid element, transmitting to the structure of the aerodyne the entirety of the forces which it receives, causing the destruction of the aerodyne or the destruction of the shock absorber, next to that of the aerodyne.
Furthermore, shock-absorber jacks are known, which can bring the undercarriages either into retracted position when the aerodyne is in flight, in lowered position for landing and taxiing, as well as into extra-low position to facilitate storage of the aerodyne in a hangar, accessibility to certain parts of the aerodyne, anchoring it in its parking space, etc. The shock-absorber jacks described in French Pat. Nos. 76/03126 and 76/33261 both of which correspond to U.S. Pat. No. 4,088,286, comprise a shock absorber that can be of the type presented above, mounted at the end of a jack shaft, in which a first chamber serves as a cylinder for the shock absorber, and is, in itself, integral with a jack piston mounted to slide in a jack cylinder, with which the piston defines a chamber operating the lowering of the landing gear when it is fed with hydraulic fluid, and operating the extra-low positioning when it is emptied, with no change in the load on the shock absorber. Furthermore, a lifting piston mounted to slide in a second chamber in the jack shaft, separated from the first, and constituting a landing-gear lifting chamber, makes it possible to load the shock absorber by pulling the shaft of the latter toward the interior of the jack shaft, when this second chamber is fed with hydraulic fluid, and the lowering chamber is emptied, which brings the undercarriage into retracted position. A mechanical locking device, for example of the claw or hydraulic type, comprising a hydraulic locking valve with controlled opening, is also provided to lock the jack shaft in extended position relative to the jack cylinder, after filling the lowering chamber. In the case of a hydraulic device, an excess-pressure valve disposed at the bottom of the lowering chamber permits a prolongation of the stroke by emptying the lowering chamber in case of a crash. But this solution has the drawback that the possible stroke of the shock absorber is not used to the best in absorbing a supplementary part of the crash energy.